The present invention relates generally to semiconductor memory devices, and, more particularly, to a method and circuit for sensing data in semiconductor memory devices.
Generally, the bit lines of a semiconductor memory device, e.g., a dynamic random access memory device (DRAM), are equalized and precharged in advance of a memory access operation, e.g., a read or write operation. The sensing control nodes of respective PMOS and NMOS sense amplifiers are also equalized and precharged in advance of a memory access operation. The accuracy and speed at which these equalizing and precharging functions are performed is an important factor with respect to the overall speed and reliability of the semiconductor memory device.
FIG. 1 is a detailed circuit diagram of a presently available data sensing circuit, which includes a PMOS sense amplifier 10, an NMOS sense amplifier 12, a bit line equalizing and precharging circuit 14, and a sense amplifier equalizing and precharging circuit 16.
With continuing reference to FIG. 1, the bit line equalizing and precharging circuit 14 is comprised of an NMOS transistor T1 whose channel is connected between complementary bit lines BL and/BL, and whose gate electrode is coupled to a primary equalizing control signal EQ1; and, two NMOS transistors T2 which are connected in series between the bit lines BL and/BL, whose gate electrodes are commonly coupled to the primary equalizing control signal EQ1, and whose source electrodes are commonly coupled to a precharge voltage, V.sub.BL.
The PMOS sense amplifier 10 is comprised of PMOS transistors T3 and T4, which are connected in series between the bit lines BL and/BL. The gate electrode of the PMOS transistor T3 is coupled to the bit line/BL, and the gate electrode of the PMOS transistor T4 is coupled to the bit line BL. The source terminals of the PMOS transistors T3 and T4 are commonly coupled to a sensing control node LA.
The NMOS sense amplifier 12 is comprised of NMOS transistors T5 and T6 connected in series between the bit lines BL and/BL. The gate electrode of the NMOS transistor T5 is coupled to the bit line/BL, and the gate electrode of the NMOS transistor T6 is coupled to the bit line BL. The drain electrodes of the NMOS transistors T5 and T6 are commonly coupled to a sensing control node/LA. The capacitances C.sub.BL and C.sub.BLB represent the inherent (parasitic) capacitances of the bit lines BL and/BL, respectively.
A PMOS pull-up transistor T7 is connected in series between the sensing control node LA and a supply voltage Vcc. The gate electrode of the pull-up transistor T7 is coupled to a sensing control signal/.phi.s. An NMOS pull-down transistor T8 is connected between the sensing control node/LA and a ground or other reference voltage Vss. The gate electrode of the pull-down transistor T8 is coupled to a sensing control signal .phi.s, which is the complement of the sensing control signal/.phi.s.
The sense amplifier equalizing and precharging circuit 16 is comprised of an NMOS transistor T9 whose channel is connected between the sensing control nodes LA and/LA; an NMOS transistor T10 which is connected between the control voltage V.sub.BL and the sensing control node LA; and, an NMOS transistor T11 which is connected between the precharge voltage V.sub.BL and the sensing control node/LA. The gate electrodes of the NMOS transistors T9, T10, and T11 are commonly coupled to a secondary equalizing control signal EQ2.
In operation, the data sensing circuit depicted in FIG. 1 works as follows. Namely, in advance of a memory access operation, the primary equalizing control signal EQ1 goes high, thereby simultaneously turning on the NMOS transistors T1, T2, and T3 of the bit line equalizing and precharging circuit 14, thereby equalizing and precharging the bit lines BL and/BL to the same voltage, i.e., V.sub.BL. Subsequently, the secondary equalizing control signal EQ2 goes high, thereby simultaneously turning on the NMOS transistors T9, T10, and T11 of the sense amplifier equalizing and precharging circuit 16, thereby equalizing and precharging the sensing control nodes LA and/LA to the same voltage, i.e., V.sub.BL. Thereafter, when a memory access operation is initiated, the sensing control signal .phi.s goes high, and the complementary sensing control signal/.phi.s goes low, thereby turning on the respective pull-down and pull-up transistors T8 and T7, thus raising the voltage level of the sensing control node LA to Vcc, and lowering the voltage level of the sensing control node/LA to Vss.
Capacitances C.sub.LA and C.sub.LAB are representative of the inherent (parasitic) capacitances of the circuits coupled to the sensing control nodes LA and/LA, respectively. Ideally, these two capacitances should be the same. However, in actual practice, these capacitances can differ, thereby inhibiting the speed of operation and degrading the reliability of the semiconductor memory device. In order to illustrate the problems caused by these differing capacitances, three separate cases will be described below, in conjunction with FIGS. 2A, 2B, and 3, namely, CASE 1 in which the capacitances C.sub.LA and C.sub.LAB are the same, CASE 2, in which the capacitance C.sub.LA of the sensing control node LA is lower than the capacitance C.sub.LAB of the sensing control node/LA, and, CASE 3, in which the capacitance C.sub.LA is higher than the capacitance C.sub.LAB. The "sensing window" in FIG. 3 indicates the time period between the time the NMOS sense amplifier 12 begins its data sensing operation, and the time the PMOS sense amplifier 10 begins its data sensing operation, or vice versa.
With reference now to FIGS. 1, 2A, 2B, and 3, CASE 1 will now be described. More particularly, when the capacitances C.sub.LA and C.sub.LAB are equal, the sensing control nodes LA and/LA will be precharged to the same voltage level V.sub.BL. When a memory access operation, e.g., a read operation, is initiated, the sensing control signal .phi.s goes high, thereby turning on the pull-down transistor T8, and thus lowering (charging down) the sensing control node/LA to Vss; and, the complementary sensing control signal/.phi.s goes low, thereby turning on the pull-up transistor T7, and thus raising the sensing control node LA to Vcc. As can be seen in FIG. 2A, since the starting voltage of the sensing control nodes LA and/LA is the same at the start of the read operation, the sensing control node LA reaches full Vcc, and the sensing control node/LA reaches full Vss. Then, the NMOS sense amplifier 12 begins its operation, and shortly thereafter, the PMOS sense amplifier 10 begins its operation. Because the sensing control node LA is at full Vcc, and the sensing control node/LA is at full Vss when the operation of the sense amplifiers 10 and 12 begins, the bit lines BL and/BL are driven to full Vcc and full Vss, respectively, or vice versa, depending upon the binary value ("1" or "0") of the data read out of the selected memory cell (not shown) of the semiconductor memory device, as can be seen in FIG. 2B.
With reference now to FIGS. 1, 2A, 2B, and 3, CASE 2 will now be described, i.e., the capacitance C.sub.LA is greater than the capacitance C.sub.LAB. In this case, the the sensing control nodes LA and/LA will be precharged to a voltage level greater than V.sub.BL. Consequently, when a read operation is initiated, the sensing control node LA will be charged up to a voltage greater than Vcc, and the sensing control node/LA will be charged down to a voltage greater than Vss, as can be seen in FIG. 2A. Because the sensing control node LA is greater than Vcc, and the sensing control node/LA is greater than Vss when the operation of the sense amplifiers 10 and 12 begins, the bit lines BL and/BL are driven to voltages greater than Vcc and Vss, respectively, or vice versa, depending upon the binary value ("1" or "0") of the data read out of the selected memory cell (not shown) of the semiconductor memory device, as can be seen in FIG. 2B. Moreover, as can be seen in FIG. 3, the operation of the PMOS sense amplifier 10 begins before the operation of the NMOS sense amplifier 12, which increases the time required to perform the read operation, increases the noise level of the ground voltage terminal Vss, and can result in data read errors. Thus, as can be seen in FIG. 3, the "sensing window" with respect to CASE 2 is much larger than it is with respect to CASE 1.
With reference now to FIGS. 1, 2A, 2B, and 3, CASE 3 will now be described, i.e., the capacitance C.sub.LA is less than the capacitance C.sub.LAB. In this case, the the sensing control nodes LA and/LA will be precharged to a voltage level less than V.sub.BL. Consequently, when a read operation is initiated, the sensing control node LA will be charged up to a voltage less than Vcc, and the sensing control node/LA will be charged down to a voltage less than Vss, as can be seen in FIG. 2A. Because the sensing control node LA is less than Vcc, and the sensing control node/LA is less than Vss when the operation of the sense amplifiers 10 and 12 begins, the bit lines BL and/BL are driven to voltages less than Vcc and Vss, respectively, or vice versa, depending upon the binary value ("1" or "0") of the data read out of the selected memory cell (not shown) of the semiconductor memory device, as can be seen in FIG. 2B. Moreover, as can be seen in FIG. 3, although the operation of the NMOS sense amplifier 12 begins before the operation of the PMOS sense amplifier 10, as it should, the NMOS sense amplifier 12 latches too quickly, and the PMOS sense amplifier 10 latches too slowly, which increases the time required to perform the read operation, increases the noise level of the supply voltage terminal Vcc, and can result in data read errors. Thus, as can be seen in FIG. 3, the "sensing window" with respect to CASE 3 is also much larger than it is with respect to CASE 1.
Based on the above, it can be appreciated that there presently exists a need in the art for a data sensing circuit for a semiconductor memory device which overcomes the above-described drawbacks and shortcomings of the presently available data sensing circuit.